1. The Field of the Invention
This invention relates to a tool, in particular a thread-rolling tap, which is constructed and intended for finishing a previously created thread without cutting, in particular an internal thread, as well as to a method of manufacturing such a tool and a method of creating a thread.
2. Background and Relevant Art
Various tools are known for creating threaded structures, in particular threads for screw connections. Many of these tools comprise in their operating region cutting parts or blades with which to produce the thread by removal of material. Among these material-removing tools are, for example, thread cutters, screw taps and thread-milling tools. Another way to produce a thread is to change the shape of the part concerned without cutting. For this purpose, tools are known and in use that re-shape the part by applying pressure, in cold conditions. Among these non-cutting thread formers are the tools sometimes called thread-rolling taps. The advantage of these tools is that, as a result of the surface re-shaping and the associated compaction, the hardness of the material of which the part is made increases in the region of the threaded profile, so that the resulting screw thread is more resistant to wear and tear.
Known thread-rolling taps, in particular for producing internal threading, comprise a shank and an operating region. The shank is typically made in cylindrical form, and its end facing away from the work piece is gripped and held within the clamping chuck of a threading device. The operating region is in the part of the thread-rolling tap, which is opposite the shank.
The operating region is provided with a screw thread in the shape of a circumferential spiral, which represents a shape complementary to that of the thread that is to be created. In longitudinal section, which shows the thread in profile, the thread-rolling tap therefore exhibits alternating elevations (teeth or forming wedges or pressure lugs) and depressions (channels or recesses), which normally are at equal distances from one another; that is, the pitch of the thread turns is constant. The teeth or forming wedges are designed so that, in cross-section, they taper outward substantially radially, at an acute angle. The cross-sectional shape of a thread turn on the tool is ordinarily a polygon.
The basic structure of a conventional thread-rolling tap is shown in FIG. 1 and explained below. A thread-rolling tap of this kind is known, for example, from DE 199 58 827 A1. Such thread-rolling taps customarily produce a screw thread in that they are inserted into a bore that is already present, by using appropriate force to push the tool forward with the operating region in front, while rotating it about the long axis of the tool shank. In this process, the forming wedges of the thread-rolling tap are pressed into the surface of the work piece, i.e. of the bore therein. The material of which the work piece is made is thus forced substantially radially into the recesses within the operating region of the thread-rolling tap. In order to facilitate penetration of the first thread turns into the work piece surface, i.e. the interior wall of the bore, the operating region of the thread-rolling tap tapers toward the tip of the tool. The forming wedges in the tapering section of the operating region, also called the thread-forming part or portion, cause the deformation of the work piece that produces the screw thread.
The documents WO 02/094491 A1 or DE 103 18 203 A1 disclose non-cutting thread-forming tools, and methods for creating screw threads without cutting, are based on another operating principle, and can be called circular thread formers (tools) or circular thread forming (methods). These thread-forming tools comprise operating regions with one or more annular circumferential profile(s) or ridge(s) separated from one another by annular grooves. Each circumferential profile is designed with a polygonal center having several polygon corners, which serve as forming wedges; such a profile is oriented perpendicular to the axis of rotation of the tool, with no pitch. In addition, axially oriented flutes can also be provided between the individual pressure lugs on the outer surface of the tools, through which cooling fluid can be supplied. The tool is introduced, while being rotated about its own axis, into a bore with diameter larger than that of the tool, and makes a circular movement along the circumference of the bore while simultaneously being pushed forward into the bore, and thus forms the screw thread in the bore without cutting.
In the case of such thread production, employing thread-rolling taps or circular thread formers, the thread is created entirely by rolling or forming by means of the forming wedges. In the process, these forming wedges are subjected to severe stress, and hence to a high degree of wear and tear.
A known means of reducing this stress, therefore, is to prepare an initial thread in a prior stage of processing. This known processing is as follows: before creation of the final thread by rolling with the forming wedges, recesses have already been produced in the bore (i.e., work piece) surface. These recesses already show recognizable signs of the thread that is to be created, but do not yet exactly correspond in their dimensions, in particular their depth, to the cross-sectional shape of the finished thread, in particular the depth of its recesses. This means, in particular, that the outside diameter of the initial thread is smaller than that of the finished thread. The previously created thread is then finished by means of a thread-rolling tap to create the final thread; that is, forming wedges are consecutively pressed into the ground of the previously created recesses.
The prior preparation of the thread can be carried out with a tool that produces chips or shavings, so that, in this two-stage method of thread creation, the advantages of cutting and non-cutting processes can be combined.
Such a method of thread creation is known, for example, from DE 196 49 190 C2. In this method the screw thread is created by means of a tool that produces a recess by both removal and rolling of material, in which process the flanks of the thread are formed accurately in both profile and dimensions by removal of material; and, at the bottom of the thread, the diameter is such that the recess floor can be compressed to the pre-specified final diameter by means of a thread-rolling tool. Solidified threads, in particular internal threads, can be created with this method.
Known thread-rolling taps that are used for finishing an initial, previously created thread comprise a thread-forming portion with forming wedges along a spiral curve, the cross-sectional shape and dimensions of the wedges being in each case identical, so that the wedges differ from one another solely in the radial distance, separating their tips from a longitudinal axis of the tool. In this case, the radial distance of the wedge tips increases in the direction opposite to that in which the tool is pushed forward; that is, the forming wedges that act later penetrate more deeply into the surface of the work piece than the preceding wedges have done. This radial shift of the forming-wedge cross-section according to the state of the art is shown, for example, in FIG. 2, and is explained below.
In these known designs of thread-rolling taps for finishing a previously created thread, the positioning of the forming wedges near the tool tip in the recess of the previously created thread is not exactly specified, but rather exhibits a degree of freedom along the axis of the tool. The reason—as is also evident in FIG. 2—is that the forming-wedge diameter is smaller than the recess diameter at a corresponding height above the tool axis. The result is that, in particular, the first forming wedge(s) (i.e. that or those closest to the tip of the tool) is/are pressed into the surface of the previously created thread recess, not centrally, but rather with a lateral offset. In this case, one of the flanks of the forming wedge is apposed to one flank of the previously created thread